Circuit Emulation Service (“CES”) allows time division multiplexing (“TDM”) services such as DS-n and E-n circuits to be transparently extended across a packet network. With circuit emulation over IP, for example, TDM data received from an external device at the edge of an Internet Protocol (“IP”) network is converted to IP packets, sent through the IP network, passed out of the IP network to its destination, and reassembled into a TDM bit stream. One application of CES is the interconnection of enterprise private telephone networks at different sites. For example, CES over a packet network can be used to connect two private branch exchanges (“PBXs”) on two different campuses without having packet transport capabilities on the PBXs themselves. This inter-working allows voice traffic between the two campuses to use a packet network backbone instead of leased TDM lines, and also allows voice and data traffic to use the same packet network.
In order for CES to function properly it is desirable to achieve the same clock in both the transmitting and receiving ends of a TDM circuit from end-to-end such that, for example, the T1 stream of a downstream PBX transmits with the clocking characteristics as the T1 stream of the upstream PBX. Known clocking techniques include both synchronous and asynchronous clocking modes, of which the asynchronous clocking modes include Differential Clock Recovery, Independent Clocking, Clock Recovery using Simple Timestamps, Adaptive Buffer-Fill-based Clock Recovery, and Adaptive Packet Inter-arrival Time Averaging-based Clock Recovery.
Differential clock recovery is typically used when there are multiple independent source clocks in a network and there is a requirement that interfaces that are already synchronized to the different source clocks be allowed to inter-work but still maintain their timing traceability to their individual source clocks. As an example, differential clocking can be used when a packet network is already synchronized to a reference source, the network clock, and then a service interface such as a PBX interface that receives its clocking from another reference source, the service clock, is connected to the packet network. To avoid resynchronizing the packet network to the service clock in the course of TDM data transport through the packet network, differential clock recovery allows the receiving service interface, i.e., the receiver, to recover the transmitting interface, i.e., the transmitter, service clock using the common network clock of the packet network available at the transmitter and receiver service interfaces.
Referring to FIG. 1, CES can be supported by a synchronous residual timestamp (“SRTS”) method of differential clock recovery where a TDM transmitter such as a PBX (100) communicates with a TDM receiver such as a PBX (102) via a packet network (104). A service clock timing signal fsc is provided to the edge PBXs (100, 102) independently from the packet network. Inter-working functions (“IWFs”) (108, 110) are driven by a network clock fnc. In order for PBX (100) to send TDM data to a user at PBX (102), the differential clocking works as follows. At IWF (108), the user introduces TDM traffic into the packet network according to the service clock signal fsc. As IWF (102) segments the TDM bit stream into packets, it measures the difference between the service clock fsc which drives it and the network clock fnc. Then, as IWF (102) generates packets it incorporates this time difference or residual time stamp (“RTS”) value into every eighth packet. The packets are then propagated through the packet network to IWF (110). As IWF (110) receives the packets, it assembles the packets into the original TDM bit stream and also reconstructs the user service clock timing signal fsc from the RTS value carried within every eighth packet. IWF (110) reconstructs the clock by adjusting the network clock fnc by the RTS. Thus, during SRTS clocking the TDM traffic is synchronized between the ingress (segmentation) side of the circuit emulation and the egress (reassembly) side of the circuit according to service clock signal fsc, while the packet network continues to function according to clock fnc.